1. Technical Field of the Invention
The present invention relates to a communications system operable to carry communication signals between satellite transceivers and earth based transceivers, and, in particular, to a system including transceivers operable to communicate with a selected one of two or more carrier protocols used in various types of cellular communications systems.
2. Description of Related Art
Terrestrial cellular communications systems are now commonly available for subscriber use in most of the major urban areas of the world. However, in spite of the success of such systems and the increasing demand for expanded coverage, there remain areas of the world where terrestrial cellular communications systems do not present a commercially viable communications option. For example, rural areas with low population densities do not offer a substantial enough potential subscriber base to justify the cost of installing a terrestrial cellular communications system infrastructure. Notably, these rural areas sometimes lack conventional wireline (fixed) telephone service for the same economic reasons.
There also exist capacity concerns with respect to existing terrestrial cellular communications systems. The dramatic success and acceptance of this form of communications has led to increased subscription growth. With more and more users, some existing terrestrial cellular communications systems have become overloaded during peak use times. While the demand justifying expansion is present, some service providers have been slow to respond to increasing demand and have not made expanded or made plans to expand the existing infrastructure to support the new demand. In some instances, governmental restrictions have hampered effective service provider response to increases in demand. In other instances, the development of terrestrial cellular technologies for handling the increased demand for cellular services has not produced the promised results as quickly as had been hoped.
The foregoing concerns have contributed towards the development of satellite cellular communications systems. Such systems rely on communications satellites placed in orbit above the surface of the earth to relay telephonic communications between cellular devices capable of originating or terminating two-way communications (also referred to as "user terminals") with satellite based transceivers. One benefit, from both a service and cost perspective, provided by a satellite cellular communications system is that a single satellite is capable of servicing the communications needs of many users distributed over a substantial service area including both rural and urban areas. In fact, one satellite may be capable of providing communications service throughout an entire country. Satellites are further capable of providing communications service in remote areas of the world where installation of conventional terrestrial cellular and wireline communications system infrastructure is difficult if not impossible.
While satellite based communication systems may solve problems related to providing communication services in remote areas, it is generally accepted that it is preferable to use a terrestrial network if one is available. Reasons for this relate primarily to service quality. Terrestrial cellular communications systems and the satellite cellular communications system, however, operate in accordance with different protocols and communications standards. Furthermore, many types of terrestrial cellular communications systems exist, again each operating in accordance with different communications standards and protocols. Thus, for example, a subscriber wishing to access and use the Asia Cellular Satellite (ACeS) or SATCOM type satellite cellular communications system must possess and utilize a mobile station configured for operation in accordance with the ACeS communications standard. If that same subscriber wanted to access and use the Global System for Mobile (GSM) communications type terrestrial cellular communications system, their mobile station must be configured for operation in accordance with the GSM communications standard. Similarly, the mobile station must be appropriately configured for operation in any other desired terrestrial or satellite cellular communications system (such as, an I-CO type satellite cellular communications system, or the Advanced Mobile Phone System (AMPS or D-AMPS) type or Personal Communications System (PCS) type terrestrial cellular communications systems).
Recent developments in mobile station design have made multi-mode mobile stations available for subscriber use. Such multi-mode devices are capable of being configured for operation in any selected one of two or more available cellular communications system types. For example, the mobile station may be configured for operation in one mode in accordance with the GSM communications standard, and in another mode in accordance with the PCS communications standard.
With the advent of satellite cellular communications systems, increasing numbers of mobile stations are capable of operation in one of the terrestrial cellular communications systems, such as GSM, and one of the satellite cellular communications systems, such as ACeS. Access to such a mobile station, in conjunction with the placement into operation of both terrestrial and satellite cellular communications systems, advantageously provides the subscriber with access to communications services substantially throughout the entire world.
Currently, a mobile station or other system operable to originate or terminate two way communications with satellite based transceivers (collectively "user terminal" or "UT") receives wide band downlink signals and transmits narrow band uplink signals. Narrow band is defined herein as being 50 kilohertz or less and wide band is defined as 200 kilohertz or greater. While some of the dual purpose user terminals described above are capable of wide band communications with terrestrial networks, by way of example, GSM, they are designed to communicate with satellite transceivers in a narrow band when operating as a user terminal. A reason that mobile stations functioning as a user terminal cannot transmit to a satellite based transceiver in a wide band is that legal regulations preclude the average transmission power level for mobile stations above a specified amount. This specified amount is a value which does not allow wide band transmission which are sufficiently strong to reach a satellite receiver. A mobile station operating as a user terminal must, therefore, transmit in an uplink to a satellite transceiver in a narrow band mode to increase the average power of the transmitted signals. The narrow band mode of transmission is necessary so as to overcome the significant amount of attenuation due to the atmosphere and distance to the transceiver. Additionally, even without the legal restrictions, the limited power of a mobile station precludes wide band transmissions with sufficient power to reach a satellite sufficiently clearly.
Currently, many existing user terminals transmit in an uplink to the satellite transceiver at a very narrow bandwidth, namely 5 kilohertz. Examples of user terminals that currently transmit in such a narrow bandwidth for the uplink portion of the two way communications include mobile stations, automobile installed communication devices, and even large roof mounted communication devices used by shopping malls, high rise buildings, hospitals, etc.
While user terminals originate or terminate a communication signal with a satellite transceiver transmit in a narrow band mode in the uplink, land earth stations (LES) are designed to transmit in an uplink in a wide band mode of operation. This is important because LES's are operable as an interface between a satellite transceiver and a terrestrial communication system and have high throughput requirements.
Despite the fact that current user terminals all transmit in a narrow band uplink, it is advantageous for a user terminal to communicate with a satellite transceiver in the uplink in a wide band mode in many cases. Because satellite transceivers communicate in a down link in a wide band mode, communication balance and symmetry may be better achieved if communication links on the uplink also are in a wide band communication mode. Such balance simplifies communication timing problems in certain situations and improves overall communication performance. Current user terminals are not designed, however, to communicate with satellite transceivers in a wide band mode of operation.
Even if a user terminal is designed to transmit in a wide band mode in an uplink to a satellite transceiver, it is preferable in some situations for the user terminal to transmit in a narrow band mode for the uplink.
One situation in which it might be advantageous for a communication device to transmit in a narrow band mode in the uplink, for example, is to improve signal reception quality by the satellite transceiver. Signal reception quality may be enhanced by reducing the data rate to increase the average amount of power for a defined signal quantity. For some communication systems, by way of example, a satellite communication system, it is known that signal path attenuation is sufficiently large that a mobile station is required to communicate in a narrow band communication mode for the signal to reach the satellite transceiver.
Stellar communication networks typically include a controller for channel management. Because such a controller is usually designed to allocate channels in an efficient manner as a part of its channel management, the controller is adapted to determine what communication links are available and to assign the channels as needed. Given the continuing need for limited carrier resources, however, there may be times in which it is desirable for a user terminal capable of communicating in wide band to be set up for communication on a narrow band carrier. By way of example, if the only available carrier (or channel) is a narrow band carrier, an earth based station which is adapted for wide band communication would be denied access until such a time that it attempts to establish a communication link and a wide band channel is available. There is a present need, therefore, for a user terminal that is capable of selectively communicating with a satellite transceiver in a narrow band mode or in a wide band mode of communications. There is also a need for communication control systems, in general, to selectively control the communication protocol and bandwidth used by the earth station for its uplink either to a base station or to a satellite transceiver.